The present invention relates to an optical communication network such as an optical LAN (Local Area Network) using optical communication.
In the communication network, optical fibers led from terminal devices are coupled with one another through couplers. Terminal devices communicate with each other through the optical fibers and couplers.
Of various types of known couplers, a so-called star coupler has been incorporated into an optical LAN disclosed by E. G. Rawson in his paper "Fibernet: Multimode Optical Fibers for Local Computer Networks", IEEE Transactions on Communications, Vol., COM-26, No. 7, July 1978.
FIG. 8 is a diagram schematically showing an optical LAN using a star coupler. A signal from each node 51 is converted into a light signal by a light emitting element 52a, and is supplied through an input optical fiber 53a to a star coupler 54. The light signals transmitted are all mixed in the coupler 54, and then are distributed through output optical fibers 53b to light sensing elements 52b. By the light sensing elements, the light signals are converted again into electrical signals, which in turn are supplied to the respective nodes 51. The optical LAN thus arranged and operated has the function (broadcasting function) capable of transmitting a signal emitted from one node to all the nodes. Therefore, it can provide a communication network similar to the ordinary LAN using coaxial cables.
A so-called multi-channel LAN has been proposed, in which each node is connected by two or more communication lines. In Japanese Patent Unexamined Publication No. Hei. 3-270432 filed by the assignee of the present Patent Application, in order to transmit real-time communication signals including speech signals and image signals, two types of buses, which are selectably operated according to the type of data to be transmitted, are used; one is of the line competition type, such as CSMA/CD (carrier sense multiple access/collision detection) and the other is of the time division type, such as TDMA (time division multiple access).
One of the approaches to realize the multi-channel LAN in optical communication is as shown in FIG. 9. In the figure, a line-competition type bus (first channel) and a time-division type bus (second channel), which are led from a bus controller 55, are connected through two star couplers 56 and 57 to nodes 58.
As shown in FIG. 10 the first channel is provided with a light emitting element 59a for signal transmission and a light sensing element 60a for signal reception. The second channel likewise is provided with a light emitting element 59b for signal transmission and a light sensing element 60b for signal reception.
The number of required star couplers in the multi-channel LAN increases as the number of buses, or the channels increases. The LAN containing an increased number of channels is complicated in construction, and suffers from an increase of cost to construct the LAN.